The growing utilization of automobiles greatly adds to the atmospheric presence of various pollutants including oxides of nitrogen and greenhouse gases such as carbon dioxide.
Internal combustion engines create mechanical work from fuel energy by combusting fuel in a thermodynamic cycle consisting (in part) of compression, ignition, and expansion. The cycle results in the travel of one or more cylindrical pistons back and forth in a cylindrical combustion chamber. Each piston is typically connected to a crankshaft that converts the linear back-and-forth motion of the piston(s) into a unidirectional rotary motion that can be used to power a vehicle. Because torque is produced only during the expansion phase, and in fact torque is absorbed during the compression phase, there are large cyclic fluctuations in torque throughout each cycle.
The cyclic, fluctuating nature of the torque produced on the crankshaft tends to favor engines with many pistons operating at high speeds. During each cycle of each piston, the piston-crankshaft assembly and the cylinder walls bear the force of the expanding combustion products. The force on the connecting rod, which is converted to torque via the crankshaft, can be resolved into a force in the direction of piston travel and a side force acting on the cylinder wall and, hence, on the engine block. These piston and side forces vary greatly during successive portions of the cycle, resulting in large fluctuations that manifest themselves either as cyclic variations in crankshaft torque or as inertial engine movement especially when the torque is taken from the shaft. The inertial movement must be resisted by the engine mounts and is ultimately transmitted to the vehicle. The key concern is the peak-to-valley amplitude of the variation. To some extent, the peak-to-valley variation in crankshaft torque can be minimized by transmitting the power through a flywheel, but inertial engine vibration is still a problem. If multiple cylinders are present, the peak-to-valley variations in both crankshaft torque and inertial engine movement can be reduced by staging and timing the combustion cycle for each piston so that their relative torque production and relative motions in their respective portions of the cycle cancel out much of the variation. The more pistons involved, the smaller the peak-to-valley amplitude of the remaining variation. The problem is exacerbated when operating at low speeds, because any variation that remains has a longer period and is more noticeable. For these reasons, most internal combustion engines used in automobiles have from four to eight pistons and operate at high speeds, typically 800 to 4000 rev/min.
Minimizing the number of pistons in an engine and operation at low speed are very attractive from an efficiency standpoint. Few-cylinder engines are simpler in construction and therefore less expensive than many-cylinder engines. More importantly, they are lighter and smaller than many-cylinder engines, allowing reductions in engine weight and engine compartment size that translate into lower curb weight and better fuel economy. Many hybrid powertrain schemes call for unusually slow engine operation (perhaps 500 rpm or less). However, the prior art does not permit such engines to operate at a low speed and high load factor without invoking the problems discussed above.
Opposed-piston or "boxer" engines have existed for some time. They are mechanically balanced, characterized by pairs of opposed pistons in which each pair is arranged in linear opposition with a crankshaft inbetween, but are not torque balanced. Because the pair is connected, one piston head may be in the expansion stroke while the other is in compression, or both may be in the same phase, but their movement is always synchronized. As long as there is an even number of piston heads, the opposition of each pair theoretically cancels out an inertial vibration. However, because the conventional "boxer" engine is not torque balanced, when power is taken from the shaft there is still a tendency to spin the engine, which must be resisted by the engine mounts and vehicle frame, and any cyclic peak-to-valley torque variation must also be borne by the mounts.